1. Technical Field
Generally, the present disclosure relates to smart electronic systems including a sensor structure, such as a temperature sensor, in combination with a dedicated integrated circuit that is connected to the sensor structure and that is configured to control power supply, data processing and the like of the entire electronic system.
2. Description of the Related Art
Immense progress has been made in the field of electronics due to the fact that sophisticated manufacturing techniques in the field of semiconductor industry allow the fabrication of circuit elements, such as transistors, having extremely small critical dimensions, thereby also providing for extremely high packing density in sophisticated semiconductor devices. As a consequence of the progress made in the semiconductor technologies circuit functions can be implemented into a single carrier material, such as a silicon substrate, so that entire circuit systems can be fabricated on a single semiconductor die. Similar progress has been made in the field of semiconductor-based sensor structures, since many of the manufacturing techniques used and developed in the semiconductor industry may also advantageously be applied to the fabrication of semiconductor-based sensors. Moreover, in a further attempt to reduce the overall volume of complex electronic systems different carrier substrate may be combined into a single package, thereby increasing the overall volume density of an electronic system with respect to the volume of a dedicated device package or substrate. Since very complex circuit portions and sensor structures can be fabricated on the basis of volume production techniques, the total cost per individual device has been significantly reduced in the past, thereby allowing the application of complex electronic systems in a wide variety of technical fields and circumstances. The low production costs may even enable the fabrication of disposable electronic systems, which may have a very limited time of usage while on the other hand producing valuable data and information with respect to a plurality of applications, however, without significantly contributing to the overall cost of ownership of specific applications.
Generally, a corresponding smart system may comprise one or more sensors, which can generate an output signal whose variation depends on parameters to be monitored, such as temperature, pressure, magnetic field, humidity, and the like, wherein dedicated electronic circuitry receives the sensor signal and provides the resources to at least preprocess the sensor signal so as to obtain information or data that is stored in the smart system or that is frequently communicated to an external device for further usage or processing.
FIG. 1a schematically illustrates a top view of a smart electronic system 100, which comprises one or more sensors 110 that are operatively connected with a dedicated integrated circuit 130 via any appropriate connection 102. The integrated circuit 130 is typically referred to as an ASIC (application specific integrated circuit) and provides for the interface capabilities for connecting to the one or more sensors 110 and has also implemented therein functions for the entire electronic system 100 in order to generate the desired data and information. Typically, the integrated circuit 130 comprises an energy management unit 170 that provides energy for operating the system 100. To this end, the circuit portion 170 may comprise any appropriate transducers, energy storage elements, and the like in order to convert externally applied energy, such as mechanical energy, heat, radiation energy, and the like, into electric energy, while in other cases in addition to or alternatively to energy converters also appropriate energy storage elements, such as batteries, and the like may be provided so as to power the electronic system 100. Moreover, a circuit portion 150 is implemented so as to act as an analog interface, also referred to as analog front end (AFE), in which an appropriate analog signal processing is accomplished on signals received from the sensor 110 via the connection 102. Furthermore, the electronic circuit 130 comprises a control unit 140, typically implemented by digital logic possibly in combination with a memory area, analog/digital and digital/analog converters, and the like, for performing the overall function of the electronic system 100. Furthermore, the control unit 140 is typically configured to control operation of the various circuit portions the electronic circuit 130. Moreover, a communication channel is frequently provided for instance in the form of a radio frequency (RF) transmitter/receiver 160, which is connected to an antenna 180 via any appropriate connection 103, thereby establishing a wireless communication channel. Consequently, the electronic system 100 can communicate with any external device by means of the wireless communication channel, thereby offering superior flexibility for using the electronic system 100 in various applications. It should be appreciated, however, that a communication channel to a peripheral device may be implemented in addition to or alternatively to the wireless communication path on the basis of a wired communication channel, if considered appropriate. In other cases the electronic system 100 may be provided without any communication resources for communicating with peripheral devices, when, for instance, the system 100 itself is appropriately configured to respond to any sensor signals obtained from the sensor 110 by using appropriate circuit portions (not shown), which may comprise electromechanical actuators, power electronics, and the like.
In many applications the electronic system 100 is developed for very restricted conditions, for instance as a disposable system in healthcare applications, in applications, in which compact organic substrates are used in combination with an electronic system, wherein frequently the electronic system, such as the system 100, is to be operated on the basis of a reduced power consumption order to enhance overall flexibility, reduce cost of ownership and provide for a desired long usable lifetime, for instance when used as a stand-alone system for specific monitoring applications, such as monitoring environmental conditions, and the like. For this reason, the electronic circuit 130 is typically designed in view of low power consumption, while at the same time providing for a desired degree of noise immunity, accuracy with respect to signal and data processing, and the like. To this end a plurality of highly sophisticated semiconductor manufacturing technologies are available. Similarly, the one or more sensors 110 may also significantly contribute to the overall power consumption of the system 100, thereby making desirable dedicated sensor structures in order to achieve the desired sensitivity, without unduly increasing the overall power consumption of the electronic system 100. Since resistive sensor structures may readily be implemented into a semiconductor-based carrier material, a Wheatstone bridge-like sensor structure is one of the most frequently used type of sensors in integrated semiconductor devices, such as the electronic system 100.
FIGS. 1b to 1e schematically illustrate circuit diagrams of different Wheatstone bridge architectures and the resulting output voltages for a given supply voltage VB of the resistive bridge.
FIG. 1b schematically illustrates the case in which three of the four bridge resistors are non-varying resistors, while a fourth resistor is considered as a varying resistor, whose resistance value is influenced by a certain parameter, such as temperature, length distortion, and the like.
FIG. 1c schematically illustrates the bridge architecture in which two resistors of oppositely arranged bridge legs are considered as varying resistors, while the remaining resistors have a substantially constant resistance value. As shown, in this case the resulting output voltage is twice the output voltage of the architecture having one varying resistor.
FIG. 1d schematically illustrates the architecture, in which also two resistors are varying resistors that are arranged within the same bridge leg and that is an opposite sign of the change of resistance value when exposed to the same influencing parameter. Also in this case the output voltage is higher compared to the case, in which only one varying resistor is provided.
FIG. 1e schematically illustrates the architecture, in which the resistance values of the four bridge resistors vary in such a way that the resistance values of two oppositely arranged resistors vary in opposite direction, thereby obtaining an output voltage that is basically four times the output voltage obtained from the bridge having only one varying resistor.
Generally, in the present application a “varying resistor” is to be understood, compared to a non-varying resistor, as a resistor having a resistance value that changes under the influence of the certain environmental parameter, such as temperature, and the like, by at least twice the magnitude compared to the non-varying resistor. For example, in a resistive structure in which the change of the resistance value of one of the resistors is to be induced by generating a length distortion, for instance by positioning the resistor on a flexible membrane, this resistor is considered as a varying resistor compared to another resistor, which is formed on a more rigid portion of the substrate material so that upon any mechanical influence on their flexible membrane the resulting distortion and thus resistance change of the “varying” resistor is at least twice the change of the non-varying resistor. Similar criteria also apply to other influencing parameters, such as temperature, and the like.
FIG. 1f schematically illustrates a bridge circuit comprising the resistors R1, R2, R3 and R4, which results in an output voltage as indicated in equations (1):
            V      0        =                                        R            ⁢                                                  ⁢            1                                R            ⁢                                                  ⁢            2                          -                              R            ⁢                                                  ⁢            3                                R            ⁢                                                  ⁢            4                                                [                      1            +                                          R                ⁢                                                                  ⁢                1                                            R                ⁢                                                                  ⁢                2                                              ]                ⁡                  [                      1            +                                          R                ⁢                                                                  ⁢                3                                            R                ⁢                                                                  ⁢                4                                              ]                                V      0        =                            R          ⁢                                          ⁢          1          ⁢          R          ⁢                                          ⁢          4                -                  R          ⁢                                          ⁢          2          ⁢          R          ⁢                                          ⁢          3                                      (                                    R              ⁢                                                          ⁢              1                        +                                                  ⁢                          R              ⁢                                                          ⁢              2                                )                ⁢                  (                                    R              ⁢                                                          ⁢              3                        +                          R              ⁢                                                          ⁢              4                                )                    
Consequently, at balance, i.e., for an output voltage V0=0, the condition as described by equation (2) is to be met.
      V    0    =            0      ⁢                          ⁢      IF      ⁢                          ⁢                        R          ⁢                                          ⁢          1                          R          ⁢                                          ⁢          2                      =                  R        ⁢                                  ⁢        3                    R        ⁢                                  ⁢        4            
If, for instance, a substantially temperature independent output voltage is to be obtained from the bridge of FIG. 1F, a variation of the resistance value of any of the bridge resistors is determined by a certain parameter of interest, while the temperature induced variation is identical for each of the bridge resistors R1, . . . , R4. For example, in a linear approximation the temperature dependence of the resistance values can be described by equation (3):R=f(T)→R(T)=Ro[1+(a/1e6)(T−To)]                a=Temperature Coefficient of Resistance (TCR, ppm/K),so that the same temperature coefficient has to be implemented in each resistor of the bridge to obtain a temperature independent sensor signal with respect to the varying parameter of interest, which may be caused by, for instance, a deformation of one or more of the resistors R1, . . . , R4. On the other hand, when using the bridge as a temperature sensor, a different temperature sensitivity has to be implemented for at least one of the resistors in order to obtain a temperature dependent sensor signal, since then the temperature dependency as described in equation 3 results in an appropriate output signal, as long as the variation of at least one of the resistors is sufficiently different from one or more of the other resistors. For example, frequently two resistors of the bridge are implemented so as to have a negative temperature coefficient compared to the remaining two resistors, thereby obtaining a maximum difference in the resistance value and thus the output voltage for a given change in temperature.        
Typically, a certain resolution over a desired operating range of the sensor is desirable in order to obtain a precise measurement within the specified operating range. As can be seen from the above cited equations, however, the resolution, i.e., the change in voltage of the bridge relative to a certain change of the parameter of interest, depends on the ratio of the change of resistance and the total resistance so that the output voltage is reduced for a high total resistance of the bridge for a given delta of the resistance value as caused by a variation of the parameter of interest. Since the change of the resistance value may be moderately small for a given change of a parameter to be monitored, a relatively small total resistance value of the bridge sensor is desirable. On the other hand, with regard to low power consumption, however, the current across the bridge is to be reduced for a given bridge voltage, which can be achieved by employing moderately high resistance values of the bridge resistors. Consequently, in low power applications typically a compromise is to be made between the resolution and thus low intrinsic noise level and the power consumed by the sensor structure. Therefore, frequently low-power sensor structures are not sufficiently sensitive for certain applications, for instance for temperature in a very limited temperature range, for instance between 30 and 45° C. for typical temperature coefficients associated with typical semiconductor-based materials as are used for the fabrication of semiconductor bridge sensors. Also in other cases moderately high resistance values may generally restrict the applicability of such low-power sensor structures in many monitoring applications.
For these reasons it has been proposed to implement a further amplification chain into the circuit portion 150 (cf. FIG. 1a) which may thus increase the resulting output voltage. On the other hand, implementing an additional amplification chain may introduce additional flicker noise, thereby eventually reducing the entire sensor resolution. Furthermore, any mismatches of the resistance values of the bridge resistors may result in an offset, which in turn can saturate the amplifier chain of the circuit or may at least increase the power supply rejection ratio (PSRR) of the amplifier.
FIG. 1g schematically illustrates a circuit diagram of a typical bridge architecture, in which the bridge 110 is connected to an amplifier 151 within the circuit portion 150, thereby providing an amplified output signal, wherein the gain of the amplifier 151 may be adjusted on the basis of a gain resistor 152, while an offset correction may be achieved on the basis of an adjustable voltage source 153.
FIG. 1h schematically illustrates a typical configuration of the electronic system 100 including the sensor 110 and their connection 102 so as to connect the circuit portion 150, which in turn comprises the amplifier 151 as explained above with reference to FIG. 1G. Moreover, the output signal of the circuit portion 150, i.e., the output signal of the amplifier 151, may be supplied to the circuit portion 140 of the circuit 130, wherein the circuit portion 140 is illustrated in the form of a microprocessor. In the circuit portion 140 an appropriate data processing is performed, as is also previously discussed, and the results of the processing may be communicated by a communication channel, as is also discussed above with reference to FIG. 1A, while in the example shown information or data can be displayed on a display unit 135 depending on the overall configuration and the application of the electronic system 100. As discussed above, however, the amplifier 151 may represent itself an additional source of noise and may also require a specific adaptation of the circuit portion 150 i.e., of the analog front end of the circuit portion 130, to the specific sensor structure 110. That is, the output voltage of the amplifier 151 depends on the gain of the amplifier 151 and the bridge resistance, and also depends on the adjustable offset voltage provided by the voltage source 153. Consequently, upon designing electronic systems for different applications, which may require different types of sensor structures 110, a redesign of the circuit portion 130 is required in order to take into account the specific configuration of the sensor structure to be used for a specific application.
Therefore active bridges have been developed, in which a preprocessed signal is provided so as to avoid the adaptation of the analog front end interface of the ASIC. That is, the signal conditioning is implemented into the bridge structure in order to provide a standardized output signal, which is received by the ASIC without requiring any modification of the design of the ASIC upon changing the sensor structure. Consequently, for different types of sensor structures the same ASIC can be used.
FIG. 1i schematically illustrates a typical configuration of a basic active bridge circuit, wherein one bridge leg includes between a first reference resistor, a first variable resistor, a p-channel transistor T3, and an n-channel transistor T4, respectively, connected in series, while the other bridge leg includes a p-channel transistor T1 and an n-channel transistor T2 between a second variable resistor and a second reference resistor. Moreover, a programmable offset current source may also be connected to the power inputs of the p-channel transistors T1 and T3, i.e., to the source terminals of these transistors. Consequently, upon a variation of the variable resistors a shift of the output voltage is obtained, which may further be processed as required. Seen from another point of view, the active circuitry of the bridge structure may be considered as a first current mirror formed by the p-channel transistors T1 and T3, wherein the first current mirror is connected in series to a second current mirror formed by the n-channel transistors T2 and T4 so that a corresponding change of the resistance value and thus of the current results in a corresponding shift of the output voltage at the output node of the bridge configuration.
Although the basic configuration as shown in FIG. 1i provides for superior flexibility in combining different types of sensor configurations with the same ASIC design, however, the overall gain of the active bridge depends on the resistance values of the sensor resistors thereby requiring a specific adaptation the active circuitry in view of the sensor structure to be used. More, the transistors of the active bridge circuit have to be implemented in the same carrier material as is used for forming the bridge resistance. Additionally, the overall current consumption of the active sensor structure may be moderately high, depending on the specific values of the bridge resistors.